https://leetcode.cn/problems/swap-nodes-in-pairs/
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(head==nullptr||head->next==nullptr)
            return head;
        
        ListNode* newnode=swapPairs(head->next->next);
        ListNode*prev=head->next;
        prev->next=head;
        head->next=newnode;
        return prev;
    }
};